is it possible to create in java hashmap that keeps its initial length?

Question

I want to create hashMap

private Map<Integer,specificObject> myMap;
myMap = new HashMap <Integer,SpecificObject>(initialCapacity);

is it possible that myMap capacity will remain the same for the entire program? even if we've reached the full capacity.

Can you fill a bottle with a higher volume of water than the volume of the bottle ? Basically, no. If you want to prevent the capacity to change, you have to keep a number of objects in the HashMap lower than loadFactor * capacity. — Dici, Sep 07 '14 at 05:06
I don't want its length to be changed.. for example if I reached the capacity I'd like it to throw an exception perhaps.. — tit, Sep 07 '14 at 05:11
Is it possible to use a regular array? It's size is final and you can access/assign by index — Vince, Sep 07 '14 at 08:09

score 0 · Answer 1 · answered Sep 07 '14 at 05:19

You can write a method which checks the current size of hashmap and throw a custom exception if the size is greater than n , where n is the max capacity you want.

Now, in the code add a call to above method before you write map.put.

This will ensure that you dont add elements to map when it exceeds your custom limit.

score 0 · Answer 2 · answered Sep 07 '14 at 05:54

You can implement such a class yourself. (The obvious approach would be to create a wrapper class for HashMap that checked the map's size, etcetera each time you called put.)

However, the regular HashMap class doesn't work that way. And furthermore, the initialCapacity parameter is:

not the map's size,
not the actual number of entries¹ you can store before the map needs to be expanded, and
not exposed via an accessible variable or getter.

^{1 - That number is determined by a formula involving the initial capacity and the load factor.}

score 0 · Answer 3 · answered Sep 07 '14 at 05:55

First, the initialCapacity is not the real capacity.

When you pass 12, the capacity is 16, which means that you pass a, the system will use b >= a and mostly reaching 2^N.

Second, in hashmap, there is a factor = most num / all capacity. For example, if factor = 0.5, and the capacity is 16, when you put 8 objects, the size will extending automatically.

score 0 · Answer 4 · answered Sep 07 '14 at 07:42

While the Java standard library does not have a size limiting cache implementation, Google's Guava has such a class. Take a look at https://code.google.com/p/guava-libraries/wiki/CachesExplained#Size-based_Eviction. Cache has the same interface as a Map with additional functionality like size limiting, timeout on entries, etc that can be defined when creating the cache.

is it possible to create in java hashmap that keeps its initial length?

4 Answers4